1. Field of the Invention
The present invention relates to the field of sensing the flow of a liquid and more particularly to a highly sensitive liquid flow detector.
2. Description of the Related Art
Liquid flow detectors are often used to detect the flow of water in water heaters. Electrically operated water heaters generally are known in the prior art and have numerous common features. The most common water heater used today is a water heater having a storage tank. In this, a supply or tank of water is pre-heated by an electric heating element or combustion of fossil fuel so that hot water is available shortly after a hot-water spigot is opened. Storage-tank water heaters waste energy in that there is substantial energy lost by radiation and conduction of the hot water stored in their storage tank, sometimes 40 or 50 gallons. The radiated energy also taxes air conditioning systems. Several attempts have been made to reduce this waste, including enhanced insulation for reducing radiated heat lost and using timers to disable the heating elements during night hours.
Recently, tankless, or instantaneous, heaters have been deployed for heating water on demand. There are several major advantages in tankless water heaters. The first is, because there is no pre-heated water, there is very little energy loss due to heat radiated from the pre-heated water. Second, the tankless water heaters are smaller, requiring fewer raw materials and requiring fewer resources in shipment. Third, the tankless water heaters occupy less space in homes, apartments, retail outlets, warehouses etc.
Several U.S. patents cover various aspects of tankless water heaters. U.S. Pat. No. 3,351,739, issued to Eckman has a tankless water heater with staged energization of electrical heating elements and a high temperature cutout switch.
U.S. Pat. No. 3,795,789, issued to Malzoni has a tankless water heater with a flow switch and electric heating elements.
U.S. Pat. No. 4,604,515, issued to Davidson and U.S. Pat. No. 4,638,147, issued to Dytch et al. include a solid state switch to control electrical current to the heating elements. Dytch mounts the solid state switch on a wall of the heating chamber, thereby cooling the switch while recovering generated heat. Dytch also teaches locating a temperature sensor at the outlet of the heater.
U.S. Pat. No. 5,479,558 to White, Jr., et al describes a flow-through tankless water heater with a flow switch. The flow switch has an arm and a ball joint, but requires significant water flow to energize the flow switch.
U.S. Pat. No. 6,552,283 to Cabrera describes a flow-switch. The flow switch has a floating magnetic set of balls that have a specific gravity higher than water yet will float upwardly in a pipe when water flows, thereby coming into proximity with a magnetic switch and energizing the heating elements. The floating set of magnetic balls must be retained within the pipe to prevent them from flowing out of the water heater. Unfortunately, this requires screens within the flow of water which, in many circumstances, corrode or clog during use.
GB 471,730 to Shepherd describes a flow switch for a tankless water heater. The flow switch of this patent has a plunger in a cylinder that, when water flows, is pushed downward, activating a mercury switch to power the heating elements. This switch uses a spring to urge the plunger back into the resting mode, thereby removing power from the heating elements. The use of a spring is problematic, in that the spring can break or corrode, thereby resulting in continuous power to the heating elements.
None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed. What is needed is a liquid flow detector that detects a small amount of liquid flow and has limited components that are exposed to the liquid, reducing failure due to corrosion and clogging.